Surface combustion burners offer an excellent method for minimizing nitric oxide and carbon monoxide formation in gaseous fuel combustion, as well as providing high radiative heat transfer in commercial and industrial applications. The control of the fuel/air ratio or stoichiometry in these fully premixed systems is critical to realizing the benefits of this type of burner. To achieve control, fuel/air control techniques based on the optical emission from the burner surface have been developed in the past. Standard techniques rely on a sensor signal indicative of the amount of oxygen (O.sub.2) or carbon monoxide (CO) in the flue gas to modify the fuel input to a burner. Apparatus operating in accordance with the prior art cannot effectively control systems with multiple burners, as the emission from each burner is co-mingled in the flue. Therefore, one burner may run fuel rich while another is running fuel lean.
U.S. Pat. No. 4,927,350 discloses a method of combustion control which determines at various loads a fuel/air peak relationship for the peak infrared radiation. A desired operating fuel/air ratio is computed as the offset between the relationship and the ratio. Recalibration of the control system is later established by determining the new fuel/air peak relationship and the offset is applied to control the surface combustion burner.
U.S. Pat. No. 4,959,010 discloses an automatically regulated combustion process. In the '010 process, fuel is mixed with an oxygen containing gas in an adjustable ratio. The exhaust gas produced by the burning is exposed to ultraviolet radiation, generating positive and negative charge carriers in the exhaust gas by means of a photoelectric charge separation process. The amount or kind of the positive and/or negative charge carriers is detected to produce a measurement value which reflects the amount and/or the charge of the carriers. A control signal is derived therefrom and the mixture ratio of the oxygen contained gas and the fuel (lambda factor) is adjusted in response to the control signal in order to improve the efficiency of the combustion.
U.S. Pat. No. 5,037,291 discloses a method and apparatus for optimizing fuel to air ratio in a combustion gas of a radiant burner. The flow rate of the gaseous fuel supply is held constant and the flow rate of the air supply is adjusted to change the relative proportion of air to fuel to an optimum value. A sensor is employed to measure the intensity of the radiation emitted by the burner while the air supply to the burner is varied. From the measurements obtained, the control parameters are derived which are then applied to set the air supply flow rate to a level that results in the optimum proportion of air and fuel in the mixture.
Still another burner control apparatus that analyzes an optical sensor signal is disclosed in U.S. Pat. No. 4,934,926. A burner operating air equivalence ratio is monitored. The burner is controlled by a method that measures OH radial spectral emission intensity at a base of a flame while combustion is in process. A linear relationship between the emission intensity and actual burner operating air equivalence ratio is used to determine that ratio while combustion is in progress. The computed ratio is compared with the desired burner operating air equivalence ratio to obtain the difference therebetween. The amount of air supplied to the burner is controlled on the basis of this difference.
U.S. Pat. No. 4,927,351 discloses a method and system for controlling the supply of fuel and air to a furnace. A controller is used with a plurality of burner assemblies in the '351 apparatus, each with its own air valve for controlling the flow of combustion air. A sensor is included with each burner for determining a condition reflecting the individual performance of the separate burner assembly. The controller operates each individual air valve in response to the performance reflecting conditions sensed by the sensors.
An air/fuel ratio controller is disclosed in U.S. Pat. No. 4,913,647. The '647 controller determines and regulates fuel/air mixture to maintain a predetermined fuel/air mixture by using a known relationship between the radiation intensity ratios of selected chemical species in the products of combustion and the fuel number as the basis to adjust the proportion of fuel within the air/fuel mixture to control the burner at the desired fuel number.
The fuel combustion control system disclosed in U.S. Pat. No. 4,545,009 makes use of an oxygen sensor in the exhaust gas as well as the amount in which the fuel control valve is opened with a corresponding fuel flow rate. A compensation coefficient is calculated by the system based on the estimated fuel flow rate and the actual fuel flow rate is controlled on the basis of the compensation coefficient. An estimated excess air ratio is calculated by the system using data representing the relationship between the opening rate of the air control damper and the air flow rate. The actual fuel flow rate and the air flow rate are controlled depending on the predetermined relationship of values between the estimated excess air ratio and the desired excess air ratio.
Another method for controlling a burner operation which utilizes an optical signal for feedback is used with the apparatus disclosed in U.S. Pat. No. 4,830,601. The '601 apparatus and method continuously monitors the burner flame during combustion by means of an optical sensor. The light therefrom is subject to spectral analysis for determining the instantaneous value of an air factor in the combustion gases. The intensity of the spectra of various compounds such as oxygen (O.sub.2), carbon dioxide (CO.sub.2) and hydrogen (H.sub.2) are utilized in the '601 method. U.S. Pat. No. 4,622,004 discloses a gas burner system that includes a detector whose output signal is used by a control unit to determine the amount of carbon monoxide in the burnt gas. Whenever this concentration exceeds a certain predetermined limit, a control signal is sent to a fan or gas valve to vary the air to fuel mixture.